Introduction
Recent decades have seen a rapid increase in chronic inflammatory
disorders due to inappropriate or misdirected immune responses
accompanied by insufficient development of immune regulatory networks.
It is generally accepted that changes in environment, lifestyle and
dietary factors may play a role in the miseducation or deficient
training of the immune system.1-3 A shift away from
traditional diets rich in plant-based foods to highly processed foods is
thought to be particularly important for negatively affecting microbiome
diversity and composition, species-specific characteristics, microbial
metabolism, and immunological tolerance (Figure 1). Whilst we
acknowledge that a range of nutritional factors may play a role in
influencing immune function and immune regulation, in this review we
will focus specifically on one dietary component – fiber.
Dietary fiber is a complex dietary component, including carbohydrate
polymers and oligomers, which makes up the non‐digestible components of
food.4,5 All dietary fibers resist digestion in the
small bowel and pass into the large bowel intact but differ in their
physiochemical characteristics (e.g. solubility, viscosity, and
fermentability), which determine their functionality in the gut and to
what degree they are accessible by microbes. Most soluble fibers can be
fermented by the gut microbiota, partially or completely, dependent on
their chemical structure. Dietary fibers can be defined on the basis of
their chemical compounds, on the basis of their functional compounds, or
both. Slight differences in definitions of dietary fibers exist due to
the wide range of non-digestible fibers that occur in nature. The
European Food Safety Authority (EFSA) defines dietary fiber as
“non-digestible carbohydrates plus lignin”.6 These
include non-starch polysaccharides (NSP) (cellulose, hemicelluloses,
pectins, hydrocolloids (i.e. gums, mucilages, β-glucans), resistant
oligosaccharides, resistant starch (consisting of physically enclosed
starch, some types of raw starch granules, retrograded amylose,
chemically and/or physically modified starches), and lignin associated
with the dietary fiber polysaccharides (Table 1).
Prebiotics are often equated with dietary fibers, but only a subset of
dietary fibers qualifies as prebiotics. The term “prebiotic” was first
defined by Gibson and Roberfroid over 25 years ago as “a non-digestible
food ingredient that beneficially affects the host by selectively
stimulating the growth and/or activity of one or a limited number of
bacteria in the colon, and thus improving host
health”.7 This definition has evolved to a more
simplified version - “a substrate that is selectively utilized by host
microorganisms conferring a health benefit”.8 The
most common prebiotic fermentable fibers that have been studied for
immune health benefits to date include inulin, fructooligosaccharides
(FOS), galactooligosaccharides (GOS) and xylooligosaccharides (XOS). The
most recent definition of prebiotics also allows for non-fiber
substrates to be potentially classified as prebiotics.